a second time". The bug was caused by incompatibility of
negations elimination algorithm and PS: during first statement 
execute a subtree with negation was replaced with equivalent 
subtree without NOTs.
The problem was that although this transformation was permanent, 
items of the new subtree were created in execute-local memory.
The patch adds means to check if it is the first execute of a
prepared statement, and if this is the case, to allocate items
in memory of the prepared statement.
The implementation:
- backports Item_arena from 5.0
- adds Item_arena::is_stmt_prepare(), 
  Item_arena::is_first_stmt_execute().
- deletes THD::allocate_temporary_pool_for_ps_preparing(),
  THD::free_temporary_pool_for_ps_preparing(); they
  were redundant.
and adds a few invariants:
- thd->free_list never contains junk (= freed items)
- thd->current_arena is never null. If there is no
  prepared statement, it points at the thd. 
The rest of the patch contains mainly mechanical changes and
cleanups.


mysql-test/r/ps.result:
  Test results updated (test case for Bug#4912)
mysql-test/t/ps.test:
  A test case for Bug#4912 "mysqld crashs in case a statement is 
  executed a second time"
sql/item_cmpfunc.cc:
  current_statement -> current_arena
sql/item_subselect.cc:
  Statement -> Item_arena, current_statement -> current_arena
sql/item_subselect.h:
  Item_subselect does not need to save thd->current_statement.
sql/item_sum.cc:
  Statement -> Item_arena
sql/item_sum.h:
  Statement -> Item_arena
sql/mysql_priv.h:
  Statement -> Item_arena
sql/sql_base.cc:
  current_statement -> current_arena
sql/sql_class.cc:
  - Item_arena
  - convenient set_n_backup_statement, restore_backup_statement
  (nice idea, Sanja)
sql/sql_class.h:
  - Item_arena: backport from 5.0
  - allocate_temporary_pool_for_ps_preparing,
    free_temporary_pool_for_ps_preparing removed.
sql/sql_derived.cc:
  current_statement -> current_arena
sql/sql_lex.cc:
  current_statement -> current_arena
sql/sql_parse.cc:
  Deploy invariant that thd->free_list never contains junk items
  (backport from 5.0).
sql/sql_prepare.cc:
  - backporting Item_arena
  - no need to allocate_temporary_pool_for_ps_preparing().
sql/sql_select.cc:
  Fix for bug#4912 "mysqld crashs in case a statement is 
  executed a second time": if this is the first execute of
  a prepared statement, negation elimination is
  done in memory of the prepared statement.
sql/sql_union.cc:
  Backporting Item_arena from 5.0.